Tsuyoshi Shimizu

Slider Wear on Disks Lubricated by Ultra-Thin Perfluoropolyether Lubricants with Different Molecular Weights

In this study, the wear properties of a magnetic head slider on disks lubricated by ultra-thin perfluoropolyether (PFPE) lubricants with different molecular weights were evaluated by the continuous sliding of magnetic head sliders using the slider contact by the dynamic flying height control. Two types of PFPE lubricants (Z-tetraol and D-4OH) with different molecular weights were evaluated. Results show that the slider wear depended on the coverage of the lubricant film; i.e., the lubricant film with sufficient coverage reduced slider wear. The lubricant film with a low molecular weight (low-Mw), including a lubricant material with a Fomblin and Demnum main chain, exhibited better coverage on a diamond-like carbon surface. Sliders with a low-Mw lubricant film showed less wear than those of a high molecular weight (high-Mw), and the depletion of the low-Mw lubricant film was less than that of the high-Mw lubricant film.

Bonding of Hard Disk Lubricants with OH-Bearing End Groups

Typical lubricants for magnetic hard disks comprise the central perfluoropolyether section and the short hydrocarbon end groups bearing hydroxyl unit(s). It had been shown earlier that chemical bonding of these lubricants to the carbon overcoat of disks involves (1) dangling bonds shielded inside the carbon, (2) transfer of the hydrogen atom of the hydroxyl unit to a dangling bond site, and (3) attachment of the remaining alkoxy system, R–CF2–CH2–O·, to the carbon surface as a pendant ether unit. Dangling bonds at or near the surface react immediately with H2O or O2 in the atmosphere. It follows that, in order to bond, the hydrocarbon end group must move into crevices of the carbon film. It was postulated that the bonding rate would depend on the length of the hydrocarbon end-group, –(CH2)n–OH. The longer the hydrocarbon sector is, the faster and the more extensively the bonding would proceed. Bonding rates were examined for a set of samples differing only in the dimension of the hydrocarbon end-group. Results clearly in accordance with the postulate were obtained. The sample set included two novel lubricants, D-2TX2 and D-2TX4, with the following end-groups, –O–CF2–CH2–O–(CH2)n=2,4–OH. Excellent bonding rate, coverage, and potential anticorrosion property were revealed for these lubricants.

Study of Molecular Conformation of PFPE Lubricants With Multidentate Functional Groups on Magnetic Disk Surface by Experiments and Molecular Dynamics Simulations

The molecular height corresponding to monolayer thickness of several types of PFPE lubricants (Z-dol2000, Z-tetraol, A20H2000, ARJ-DS, ARJ-DD, and OHJ-DS) with different molecular structures was estimated from their spreading profiles. The molecular height of Z-dol2000 was smaller than that of Z-tetraol, even though these lubricants had almost equal molecular weights. This result shows that the molecular height of the lubricant with high-polarity end groups was higher than that of the lubricant with low-polarity end groups. ARJ-DD showed higher molecular height than ARJ-DS, because the polarity of the two OH end groups in ARJ-DS was lower than that of the four OH end groups in ARJ-DD. On the other hand, OHJ-DS showed a low molecular height even though it had the highest molecular weight and four OH functional groups. Thus, a lubricant molecule with a low polarity per segment of the main chain has a low molecular height and shows a low degree of conformation on a carbon surface. These experimental results agreed with molecular dynamics (MD) simulation results. Therefore, it can be hypothesized that lubricants with multidentate functional groups having strong polarities do not exhibit a flat conformation on a disk surface because the lubricant molecules form random coil shapes on account of the strong polarity. On the basis of these results, we propose a design rule for lubricant molecules to achieve flat conformation on magnetic disks, i.e., a flat molecular conformation can be achieved on a magnetic disk surface by using molecules that have multisegments with a weak polar function like as OHJ-DS.